Mammalian cell cycle progression is a tightly controlled process in which transitions through different phases are conducted in a highly ordered manner and guarded by multiple checkpoints. The retinoblastoma protein (pRb) is the checkpoint protein for the G1 to S phase transition. pRb associates with a family of E2F transcription factors to prevent their activity in the absence of appropriate growth stimuli (See Ortega et al., Biochimica et Biophysica Acta-Reviews on Cancer 2002; 1602 (1):73-87; Shapiro, Journal of Clinical Oncology 2006; 24 (11):1770-1783). Upon mitogen stimulation, quiescent cells begin their entry into S phase by newly synthesizing D-cyclins, which are the activators of cyclin-dependent kinases 4 and 6 (CDK4/6). Once bound by the cyclins, CDK4/6 deactivate the pRb protein via phosphorylation. The phosphorylation of pRb releases E2F to direct the transcription of genes required for S phase. A full deactivation of pRb requires phosphorylations by both cyclin D-CDK4/6 and cyclin E-CDK2. Phosphorylations by CDK4/6 at specific sites of pRb (Ser780, Ser795) have been shown to be a prerequisite for cyclin E-CDK2 phosphorylation. (See Lundberg et al., Molecular and Cellular Biology 1998; 18 (2):753-761) In addition to D-cyclins, the activity of CDK4/6 is regulated by p16, encoded by INK4a gene, which inhibits the kinase activity. (See Kamb et al., Science 1994; 264 (5157):436-440) The CIP/KIP proteins, which are the inhibitors of cyclin E-CDK2, also bind to cyclin D-CDK4/6 complex, and this results in further activation of CDK2 by sequestering the CIP/KIP proteins away from their target. (See Sherr et al., Genes & Development 1999; 13 (12):1501-1512) Therefore, the cyclin D-CDK4/6 is a key enzyme complex that regulates the G1 to S phase.
The D-cyclin-CDK4/6-INK4a-pRb pathway is universally disrupted to favor cell proliferation in cancer. In a majority of cases (˜80%), cancers maintain a functional pRb and utilize different mechanisms to increase the activity of CDK4/6 kinase. (See Ortega et al., Biochimica et Biophysica Acta-Reviews on Cancer 2002; 1602 (1):73-87; Shapiro, Journal of Clinical Oncology 2006; 24 (11):1770-1783)) In Mantle cell lymphoma (MCL), cyclin D1 is translocated to IgH promoter (t11:14) which results in constitutive expression of the protein, leading to activation of CDK4/6 (See Amin, et al., Archives of Pathology & Laboratory Medicine 2003; 127 (4):424-431; Oudat, et al., Modern Pathology 2001; 14 (1):175A) This translocation is observed in >90% of the MCL cases and considered pathognomic for the disease. The D-cyclin is also translocated in 20% of multiple myelomas. (See Bergsagel et al., Immunological Reviews 2003; 194 (1):96-104)
In addition to translocation, D-cyclin abundance can also be increased by amplification or overexpression, and the examples of these can be found in squamous cell esophageal cancer, where a significant portion exhibits cyclin D1 amplification (See Jiang, et al., Cancer Research 1992; 52 (10):2980-2983) and in breast cancer, where the overexpression of cyclin D1 is frequent (See Arnold et al., Journal of Clinical Oncology 2005). The CDK4/6 kinase activity can also be increased by amplification of the CDK4 gene itself and the co-amplifications of CDK4 and MDM2 genes are observed in almost all cases of dedifferentiated liposarcomas. (See Sirvent, et al., American Journal of Surgical Pathology 2007; 31 (10):1476-1489) The genetic inhibitor of CDK4/6 is also frequently inactivated in cancer to achieve CDK4/6 activation and the examples of this include non small cell lung cancer, melanoma and pancreatic cancer (Brambilla, et al., Journal of Pathology 1999; 188 (4):351-360; Cowgill et al., American Journal of Surgery 2003; 186 (3):279-286; Gazzeri, et al., Oncogene 1998; 16 (4):497-504; Kamb et al., Science 1994; 264 (5157):436-440; Ortega et al., Biochimica et Biophysica Acta-Reviews on Cancer 2002; 1602 (1):73-87).
In addition to these genetic defects directly related to the D-cyclin-CDK4/6-INK4a-pRb pathway, the activity of the CDK4/6 kinases can also be enhanced by oncogenic aberrations of the mitogen pathways that increase D-cyclin expression. The examples here include EGFR amplifications in non small cell lung cancer (NSCLC), activating K-Ras mutations in pancreatic cancer, V600E B-Raf mutation in melanoma and PTEN inactivation in colon cancer (See Dailey, et al., Cytokine & Growth Factor Reviews 2005; 16 (2):233-247; Engelman, Nature Reviews Cancer 2009; 9 (8):550-562; Garcia-Echeverria, Purinergic Signalling 2009; 5 (1):117-125, Gray-Schopfer et al., Cancer and Metastasis Reviews 2005; 24 (1):165-183, John, et al., Oncogene 2009; 28:S14-S23, Sharma, et al., Nature Reviews Cancer 2007; 7 (3):169-181).
Taken together, a large number of human neoplasms achieve enhanced cell proliferation by increasing CDK4/6 activity and a small molecule inhibitor of these kinases might provide an effective means to treat these diseases.
Inhibitors of CDKs are known and patent applications have been filed on such inhibitors. (See, for example, WO2007/140222)
Thus attempts have been made to prepare compounds that inhibit CDK4/6 activity and a number of such compounds have been disclosed in the art. However, in view of the number of pathological responses that are mediated by CDK4/6, there remains a continuing need for inhibitors of CDK4/6 which can be used in the treatment of a variety of conditions, including cancer.